Unicolonial ants possess an unusual social system characterized by the absence of internest aggression resulting in expansive networks where individuals move freely among distant nests. The Argentine ant, Linepithema humile (Mayr), can form geographically vast and numerically large unicolonial populations, or supercolonies, a trait that has been linked to its ecological success in the introduced range, and is one of the few invasive ants in which native and introduced populations have been examined to elucidate the origins and maintenance of unicoloniality. Supercolony formation may result from mixing of genetically homogenous and nonaggressive colonies, or initially aggressive colonies harbouring the most common recognition alleles. In this study, we examined interactions between mutually aggressive L. humile colonies in the absence of barriers limiting intercolony encounters to determine whether aggressive interactions result in either colony elimination or fusion into new nonaggressive colonies. By pairing experimental laboratory and field colonies displaying varying levels of intraspecific aggression, we determined that pairs that did not fuse had higher numbers of workers fighting and killed than colony pairs that fused and that genetic and cuticular hydrocarbon similarity between colony pairs was correlated with both levels of intraspecific aggression and colony fusion. We suggest that selective fusion of initially aggressive colonies sharing certain recognition cues may be a proximate mechanism shaping L. humile social structure, leading ultimately to extreme unicoloniality in introduced populations when ecological conditions are favourable. © 2007 The Association for the Study of Animal Behaviour.